1. Field of the Invention
The present invention relates to a method of producing a sintered porous anode body for a solid electrolytic capacitor and a sintering apparatus thereof, and more particularly, to a method of producing a sintered porous anode body of valve-metal.
2. Description of the Prior Art
A solid electrolytic capacitor comprises a sintered porous anode body of valve-metal such as tantalum and aluminum. A conventional sintered porous anode body is produced in such a manner that powder of the valve-metal is first molded under pressure so as to form through a pressurized molding process a molded piece which has a predetermined contour. A anode lead wire made of the valve-metal is implanted into the molded piece. The molded pieces are then placed on a sintering tray in an aligned condition or at random to be sintered in a vacuum state ranging from 10.sup.-5 to 10.sup.-6 Torr at a temperature ranging from 1500.degree. C. to 1800.degree. C. In the sintering process, in order to efficiently exhaust an impurity gas emitted from the molded pieces to attain a desired vacuum state with a lowered pressure, there has been necessitated a sintering apparatus including such large-sized exhausting machines for developing the vacuum state as a diffusion pump and a cryopump as shown in FIG. 5.
Since the valve-metal is in a state of a very fine powder, there is attained a large surface area of the metal powder particles to be formed into molded pieces. In consequence, when the valve-metal powder is sintered at a high temperature in the conventional method of sintering an anode body, the amount of the impurity gas emitted from the molded pieces is increased, making it difficult to keep a desired high vacuum of a lowered pressure. Resultantly, a solid electrolytic capacitor having the conventional sintered anode body is attended with deterioration in electric characteristics such as an increase in the leakage current or a decrease in the withstand voltage and in mechanical characteristics, for example, the lead wire is easily bent down.
Moreover, due to the necessity of the usage of large-sized exhausting machines for the vacuum, a long period of time is to be elapsed when starting and/or stopping the operation of these machines, which leads to a decrease in the operation efficiency; furthermore, due to the complexity of the configuration of the exhausting system, the maintainability is decreased and the probability of failure is increased.